The present invention is related to a zero current detector and, more particularly, for a zero current detector using the sources of transistors in the design to sense the voltage of a power transistor for use in a DC-DC converter application.
A classical implementation of a zero current detector according to the prior art is shown in FIGS. 1(a) and 1(b). A first typical implementation of a zero current detector is illustrated in FIG. 1(a) including P-channel transistors M1 and M2, resistor R1, current source I1, and N-channel diode-connected load devices M3 and M4. The state of the COMPARE signal is indicative of the direction of the current flowing through the power transistor M0. Note that the gates of transistors M1 and M2 are used to monitor terminals “A” and “B” of power device M0. The voltage on the gate of transistor M1 is designated INM and the voltage on the gate of transistor M2 is designated INP. In FIG. 1(a), Id=(Vgs2−Vgs1+I1*R1/2)/Rdson. The circuit of FIG. 1(a) is relatively easy to implement, but the accuracy is not ideal because of the mismatching of the R1 resistor value and the CMOS body effect acting upon power transistor M0. Moreover, the circuit of FIG. 1(a) cannot be used in a high voltage application. In FIG. 1(b), the accuracy of the circuit is improved due to the addition of N-channel transistor M5 and the IBIAS current source as well as removing resistor R1, but the transient response is slow because the voltage on node C must be discharged from VCC to the voltage on node B. The circuit of FIG. 1(b) is also not suitable for high voltage applications.
What is desired, therefore, is a zero current detector having high sense accuracy and high transient response, able to be used in extensive different applications, yet easy to implement.